Preheat your oven to 350°F (175°C) and grease a loaf pan. Dice the apples into small pieces and toss them with brown sugar and ground cinnamon. Set aside to absorb flavors.

In a large mixing bowl, cream together the softened unsalted butter and granulated sugar until light and fluffy, about 2-3 minutes. Beat in the eggs one at a time, followed by vanilla extract. In a separate bowl, whisk together all-purpose flour and baking powder, then gradually alternate adding this dry mixture with the milk to the butter mixture until just combined.

Pour half of the prepared batter into the greased loaf pan. Sprinkle half of the apple mixture over the batter. Spoon the remaining batter on top, smoothing it out, and layer the rest of the apples on the surface. Gently swirl the layers with a knife.

Place the loaf pan in the preheated oven and bake for 50-55 minutes. It's done when a toothpick inserted into the center comes out clean.

Once baked, let it cool in the pan for about 10 minutes. Transfer to a wire rack to cool completely. Prepare the glaze by mixing powdered sugar with milk and vanilla until desired consistency is reached, then drizzle over the cooled bread.